In 3GPP (Third Generation Partnership Project), in a radio communication system corresponding to LTE (Long term Evolution), for which the standards are being set up currently, a radio base station eNB assigns a radio resource in the radio communication between the radio base station eNB and a radio terminal UE (for example, see Non Patent Literature 1). Furthermore, in a radio communication system corresponding to LTE, either Frequency Division Duplex (FDD) or Time Division Duplex (TDD) is employed in the radio communication between the radio base station eNB and the radio terminal UE.
Moreover, in an LTE radio communication system employing TDD (TDD-LTE), each of the adjacent radio base stations eNB uses the same frequency band in the radio communication with a subordinate radio terminal UE. Therefore, null steering is being examined, which is a technique according to which at the time of calculating an antenna weight for a downlink resource block assigned to a subordinate radio terminal UE, a first radio base station eNB directs the null in the direction of a radio terminal UE subordinate to a second radio base station eNB, in order to control interference. When the first radio base station eNB performs null steering, the effect of controlling interference in the radio communication between the second radio base station eNB and the subordinate radio terminal UE is obtained. In null steering, when the first radio base station eNB receives a sounding reference signal (SRS), which is an uplink radio signal transmitted by a radio terminal UE subordinate to the second radio base station eNB, the direction of arrival of the SRS is recognized as the direction of the radio terminal UE subordinate to the second radio base station eNB. Moreover, the antenna weight is calculated such that the first radio base station eNB directs the null in the direction of the radio terminal UE subordinate to the second radio base station eNB.
When a plurality of radio terminals UE subordinate to the second radio base station eNB simultaneously transmit SRS of the same frequency band, the first radio base station eNB simultaneously receives the SRS arriving from different directions from each of the plurality of radio terminals UE subordinate to the second radio base station eNB. Therefore, it is not possible for the first radio base station eNB to determine the direction of turning the null, which makes it difficult to perform accurate null steering.